Preheat fan oven to 180°C (fan oven 160°C, 300°F, gas mark 3). Lightly grease baking sheets.
In a large bowl, cream together the butter, sugar, and egg. In another bowl, sieve and combine cocoa, flour, bicarbonate of soda, baking powder, and salt.
In a small bowl, stir the vanilla extract into the milk. Add the dry ingredients to the butter mix, alternating with the milk mixture; beating until smooth.
Drop batter using two spoons (to make 20-30 halves or 10-15 whoopies when filled with cream) onto the prepared baking sheets. With the back of a spoon spread batter into 5cm (2-inch) circles, leaving approximately 4cm (2 inches) between each cake.
Bake for 15 mins or until they are firm to the touch. Remove from oven and let cool completely on a wire rack.When the cakes are completely cool, spread the flat side (bottom) of one chocolate cake with a generous amount of clotted cream. Top with another cake, pressing down gently to distribute the filling evenly. Repeat to make 10-15 whoopies.